1. Technical Field
The present invention relates generally to the field of metrology, and, more particularly, to scatterometry metrology.
2. Discussion of Related Art
The following documents disclose various aspects of related art, and are incorporated herein by reference in their entireties.
U.S. Patent Publication No. 2008/0074666 discloses detecting both the 1st and 0th diffraction orders using a scatterometer. The 1st diffraction orders are used to detect the overlay error. The 0th diffraction order is then used to flag whether there is a false overlay error calculation of magnitude greater than the bias, but smaller than the pitch of the grating. Unfortunately, U.S. Patent Publication No. 2008/0074666 does not attempt to calculate inaccuracy.
U.S. Patent Publication No. 2013/0054186 discloses a metrology method comprising the steps of measuring scattering properties of a first target comprising a first structure and a second structure, constructing a model of the first structure using the measured scattering properties, the model comprising a first model structure corresponding to the first structure, modifying the model by overlaying the first model structure with an intermediate model structure, further modifying the model by replacing the intermediate model structure with a second model structure, corresponding to the second structure, calculating a second defect-induced overlay error between the first model structure and the second model structure, the first and second model structures being overlaid with respect to each other in the further modified model and determining an overlay error in a second target using the calculated second defect-induced overlay error. U.S. Patent Publication No. 2013/0054186 measures each grating separately, models it and finds the inaccuracy due to target imperfections.
WIPO Publication No. 2010/115686 discloses a method for the determination of an overlay error between two successive layers produced by a lithographic process on a substrate by using the lithographic process to form at least one periodic structure of a same pitch on each of the layers. One or more overlaid pairs of the periodic structures are formed in parallel, but offset relative to each other. A spectrum, produced by directing a beam of radiation onto the one or more pairs of periodic structures is measured. One or more portions of the spectrum are determined in which the relationship between the offset between the one or more pairs of periodic structures and the resultant variation in measured intensity of the spectrum at the one or more portions is more linear than the relationship outside the one or more portions. The offset between the one or more pairs of periodic structures on the basis of intensity measurements of the spectrum in the one or more portions of the spectrum is determined and used to determine the overlay error. WIPO Publication No. 2010/115686 applies the correction to future measurements, selects only a part of the spectrum, and neglects possible correlations between different pixels.
U.S. Patent Publication No. 2009/0073448 discloses the detection of the reflected radiation from a target mark including, for example, a plurality of gratings by an array of pixels. The overlay error of the gratings for each pixel is detected, and an array of overlay errors is determined. Rather than simply averaging the overlay error value for all the pixels, filtering is performed. Pixels may be filtered according to the detected value of the overlay error or the detected intensity of the pixel. U.S. Patent Publication No. 2009/0073448 uses model-based calibration which, if the imperfections are random, requires a separate model for each measurement. Further, it selects only a part of the spectrum and neglects possible correlations between different pixels.
U.S. Patent Publication No. 2011/0134419 discloses a method and associated apparatus to determine an overlay error on a substrate. A beam is projected onto three or more targets. Each target includes first and second overlapping patterns with predetermined overlay offsets on the substrate. The asymmetry of the radiation reflected from each target on the substrate is measured. The overlay error not resultant from the predetermined overlay offsets is determined. The function that enables calculation of overlay from asymmetry for other points on the wafer is determined by limiting the effect of linearity error when determining the overlay error from the function.
WIPO Publication No. 2013/079270 discloses an inspection method and corresponding apparatus enabling classification of pupil images according to a process variable. The method comprises acquiring diffraction pupil images of a plurality of structures formed on a substrate during a lithographic process. A process variable of the lithographic process was varied between formation of the structures, the variation of the process variable resulting in a variation in the diffraction pupil images. The method further comprised determining at least one discriminant function for the diffraction pupil images, the discriminant function being able to classify the pupil images in terms of the process variable. WIPO Publication No. 2013/079270 finds the process conditions of the wafers, but not the accuracy per target.
U.S. Patent Publication No. 2008/0239318 discloses a method of measuring asymmetry in a scatterometer where a target portion is illuminated twice, first with 0° of substrate rotation and secondly with 180° of substrate rotation. One of those images is rotated and then that rotated image is subtracted from the other image. In this way, asymmetry of the scatterometer can be corrected. U.S. Patent Publication No. 2008/0239318 finds the tool induced shift.
U.S. Patent Publication No. 2011/0292365 discloses methods, apparatuses, and lithographic systems for calibrating an inspection apparatus. Radiation is projected onto a pattern in a target position of a substrate. By making a plurality of measurements of the pattern and comparing the measured first or higher diffraction orders of radiation reflected from the pattern of different measurements, a residual error indicative of the error in a scatterometer may be calculated. This error is an error in measurements of substrate parameters caused by irregularities of the scatterometer. The residual error may manifest itself as an asymmetry in the diffraction spectra. U.S. Patent Publication No. 2011/0292365 uses multiple measurements in multiple measurement conditions for calibration per target.
U.S. Patent Publication No. 2011/001978 discloses a method for determining an overlay error between two successive layers produced by a lithographic process on a substrate, including using the lithographic process to form a calibration structure including a periodic structure of the same pitch on each of the layers, such that an overlaid pair of periodic structures is formed, the structures being parallel, but offset relative to each other by an overlay amount. A spectrum produced by directing a beam of radiation onto the calibration structure is measured and compared with one or more modeled spectra so as to determine values of the grating parameters for the calibration structure from the measured spectrum. The lithographic process is used to form further overlaid periodic structures on the same or one or more subsequent substrates, the determined grating parameter values for the calibration structure being used to determine overlay amounts for the further overlaid periodic structures. U.S. Patent Publication No. 2011/001978 uses modeling (such as RCWA-based modeling) of calibration targets and compares the measurements to the calculated per pixel overlay/asymmetry.
WIPO Publication No. 2011/012624 discloses a method of determining the focus of a lithographic apparatus used in a lithographic process on a substrate, the lithographic process is used to form a structure on the substrate, the structure having at least one feature which has an asymmetry in the printed profile which varies as a function of the focus of the lithographic apparatus on the substrate. A first image of the periodic structure is formed and detected while illuminating the structure with a first beam of radiation, the first image being formed using a first part of non-zero order diffracted radiation. A second image of the periodic structure is formed and detected while illuminating the structure with a second beam of radiation. The second image is formed using a second part of the non-zero order diffracted radiation which is symmetrically opposite to the first part in a diffraction spectrum. The ratio of the intensities of the measured first and second portions of the spectra is determined and used to determine the asymmetry in the profile of the periodic structure and/or to provide an indication of the focus on the substrate. In the same instrument, an intensity variation across the detected portion is determined as a measure of process-induced variation across the structure. A region of the structure with unwanted process variation can be identified and excluded from a measurement of the structure. WIPO Publication No. 2011/012624 uses sensitive features to identify focus/process-induced variations across the wafer, and not the inaccuracy in nm.
U.S. Patent Publication No. 2011/0178785 discloses a method for the calibration of an angularly resolved scatterometer is performed by measuring a target in two or more different arrangements. The different arrangements cause radiation being measured in an outgoing direction to be different combinations of radiation illuminating the target from ingoing directions. A reference mirror measurement may also be performed. The measurements and modeling of the difference between the first and second arrangements is used to estimate separately properties of the ingoing and outgoing optical systems. The modeling may account for symmetry of the respective periodic target. The modeling typically accounts for polarizing effects of the ingoing optical elements, the outgoing optical elements and the respective periodic target. The polarizing effects may be described in the modeling by Jones calculus or Mueller calculus. The modeling may include a parameterization in terms of basis functions such as Zernike polynomials. U.S. Patent Publication No. 2011/0178785 uses multiple measurement conditions to calibrate the optical system, rather than the target.
U.S. Patent Publication No. 2011/0255066 discloses an apparatus that measures properties, such as overlay error, of a substrate divided into a plurality of fields. The apparatus includes a radiation source configured to direct radiation onto a first target of each field of the substrate. Each first target (T4G) has at least a first grating and a second grating having respective predetermined offsets, the predetermined offset (+d) of the first grating being in a direction opposite the predetermined offset (−d) of the second grating. A detector is configured to detect the radiation reflected from each first target and to obtain an asymmetry value for each first target from the detected radiation. Further, a module is configured to determine an overlay value for each first target based on at least the obtained asymmetry value and the predetermined offsets and determine a polynomial fit across a plurality of first targets of a corresponding plurality of fields of the substrate for a relationship between the obtained asymmetry value and determined overlay value of each first target. U.S. Patent Publication No. 2011/0255066 uses four-cell targets to find a relation between differential signal and OVL and uses this relation to extract OVL from two-cell targets, and does not deal with target asymmetry and measurement inaccuracy.
Typically, the overlay measurements and, specifically, the scatterometry algorithm are very sensitive to process variations. Acceptable process variations (which do not affect the device performance) may be wrongly reported as unacceptable overlay errors. Therefore, there has been a long felt need for metrology methods to reduce this inaccuracy.